The present invention relates to a discharge-pumped excimer laser device, and more particularly to a discharge-pumped excimer laser device having a cross flow fan rotatably supported by magnetic bearings, for generating a high-speed gas flow between a pair of main discharge electrodes.
FIG. 9 is a cross-sectional view showing a general structure of a conventional discharge-pumped excimer laser device of the type described above. As shown in FIG. 9, the conventional discharge-pumped excimer laser device has preionizing electrodes (not shown) for preionizing a laser gas and a pair of main discharge electrodes 102, 102 for producing an electric discharge to be able to oscillate a laser beam, the preionizing electrodes and the main discharge electrodes 102, 102 being disposed in a laser chamber 101 filled with a laser gas. The laser chamber 101 also houses therein a cross flow fan 103 for generating a high-speed gas flow between the main discharge electrodes 102, 102.
The cross flow fan 103 has a rotatable shaft 104 extending from opposite ends thereof and rotatably supported by bearings 106, 106 that are mounted in opposite sides of the laser chamber 101. The laser chamber 101 has windows 105, 1051 for emitting the laser beam from the laser chamber 101 therethrough and a dust filter (not shown) for removing dust from the laser gas in the laser chamber 101.
The bearings 106, 106 by which the cross flow fan 103 is rotatably supported are lubricated by a lubricant that usually comprises a fluorine-based grease. It is known that the fluorine-based grease is least degraded by a corrosive gas such as a fluorine-based gas used in the discharge-pumped excimer laser device. However, the fluorine-based grease is problematic that it tends to be diffused in the laser gas and causes a photochemical reaction with light generated by the electric discharge and fluorine contained in the laser gas, producing impurities such as CF4, etc. which are liable to degrade the laser gas.
There has been proposed a discharge-pumped excimer laser device in which components of the bearings are coated with a solid lubricating film to dispense with any grease. However, it has been pointed out that the solid lubricant causes more friction in the bearings than the grease lubricant. In addition, since the solid lubricating film has a thickness of up to 1 xcexcm, it is likely to be peeled off when submicroscopic metal dust particles produced by the electric discharge in the laser chamber find their way into the bearings.
There has also been proposed a process of positively introducing a laser gas from which dust particles have been removed into regions between the cross flow fan and the bearings with a view to protecting the bearings. It has also been proposed to make bearing holders of PTFE (polytetrafluoroethylene) that has an excellent lubricating capability. Since, however, the fluorine-based material is used, scraped dust particles tend to be diffused into the laser chamber.
Because the discharge-pumped excimer laser device uses a halogen gas which is highly reactive with the laser gas, the laser chamber houses therein components which are made of Ni and Ni-plated metal materials that are highly resistant to halogen-induced corrosion. However, upon laser oscillation, since the laser gas is excited by the electric discharge between the discharge electrodes, the Ni and Ni-plated metal materials in the laser chamber are sputtered, thus producing an Ni powder and an Ni powder that has chemically reacted with the halogen gas in the laser gas.
Inasmuch as the Ni powder is ferromagnetic, if the contactless magnetic bearings are used as the bearings and a motor is incorporated, then the Ni powder is attached to and deposited on the magnetic material surfaces of the magnetic bearings and the motor, tending to obstruct the rotation of the cross flow fan. It has heretofore been customary to increase the clearance between the rotor and the stator as much as possible to prevent the rotation of the cross flow fan from being obstructed even when dust particles are attached to the magnetic material surfaces of the magnetic bearings and the motor.
However, as the allowance for dust particles to be attached to the magnetic bearings and the motor increases, the clearance between the rotor and the stator needs to be increased, resulting in a reduction in the force for controlling the magnetic bearings. Generally, because the force for controlling the magnetic bearings are reduced in proportion to the square of the clearance, if the clearance is doubled and the force for controlling the magnetic bearings is to be maintained, then it is necessary to use magnetic bearings in which the surface area of the electromagnet is increased four times, or the number of turns of the electromagnet is increased four times, or the coil control current is increased twice.
The present invention has been made in view of the above drawbacks. It is a first object of the present invention to provide a discharge-pumped excimer laser device which includes a laser chamber filled with a laser gas that is deteriorated to a small extent, magnetic bearings and a motor that are resistant to the entry of dust particles, and parts that are held in contact with the laser gas and suffer little damage, and has a long service life.
A second object of the present invention is to provide a discharge-pumped excimer laser device which prevents dust particles from entering magnetic bearings and a motor and can be continuously operated over a long period of time.
To achieve the above objects, there is provided in accordance with an invention described in claim 1, a discharge-pumped excimer laser device, comprising: a laser chamber filled with a laser gas and housing at least a pair of main discharge electrodes for producing an electric discharge to oscillate a laser beam; a cross flow fan having opposite ends rotatably supported by magnetic bearings, for producing a high-speed laser gas flow between the main discharge electrodes; a motor for rotating the cross flow fan; laser gas flow passages extending through gaps between rotor side and stator side of the magnetic bearings and the motor and communicating with an interior of the laser chamber; a laser gas introduction passage extending from the interior of the laser chamber and communicating with the laser gas flow passages; and filters disposed in the laser gas introduction passage.
With the above invention, the laser gas in the laser chamber flows from the laser gas introduction passage through the laser gas flow passages back into the laser chamber. When the laser gas flows through the laser gas flow passages, the laser gas flows through the gap between the stator side and rotor side of the magnetic bearings by which the cross flow fan is rotatably supported and the gap between the stator side and rotor side of the motor which rotates the cross flow fan, thus replacing the gas in these gaps. Therefore, the working time required to remove impurities from the discharge-pumped excimer laser device when it starts to operate is shortened, and the discharge-pumped excimer laser device is kept dust-free.
According to an invention described in claim 2, the discharge-pumped excimer laser device according to claim 1, wherein the magnetic bearings and the motor are accommodated in housings joined to opposite sides of the laser chamber.
With the above invention, the laser chamber and the housings are separate from each other, and can be serviced for maintenance and assembled with ease.
According to an invention described in claim 3, the discharge-pumped excimer laser device according to claim 2, wherein the laser gas flow passages extend over an entire length of the housings and communicate with the laser gas introduction passage at respective ends of the housings.
With the above invention, the laser gas is caused to flow in one direction in the laser gas flow passages over their entire length, and is prevented from being trapped in the laser gas flow passages.
According to an invention described in claim 4, the discharge-pumped excimer laser device according to claim 1, 2, or 3, wherein portions of the magnetic bearings and the motor which face the laser gas flow passages are made of a material which is resistant to corrosion by the laser gas or covered with a can made of a material which is resistant to corrosion by the laser gas.
With the above invention, since the portions of the magnetic bearings and the motor which face the laser gas flow passages are made of a material which is resistant to corrosion by the laser gas or covered with a can made of a material which is resistant to corrosion by the laser gas, the corrosion resistance of the magnetic bearings and the motor is increased.
According to an invention described in claim 5, the discharge-pumped excimer laser device according to claim 4, wherein the material which is resistant to corrosion by the laser gas is permalloy, austenitic stainless steel, nickel-copper alloy, nickel-chromium alloy, or nickel-chromium-molybdenum alloy.
With the above invention, the stator side and rotor side of the motor and the stator side of the magnetic bearings are covered with a can of austenitic stainless steel or the like, and the rotor side of the magnetic bearings are made of a pure PC permalloy, so that the magnetic bearings and the motor can have their service life extended can have their performance and efficiency increased, and can be reduced in size.
According to an invention described in claim 6, the discharge-pumped excimer laser device according to claim 1, 2, 3, 4, or 5, wherein a differential pressure generating mechanism is disposed in the laser gas introduction passage.
With the above invention, since the differential pressure generating mechanism is disposed in the laser gas introduction passage, the laser gas is caused to flow reliably from the laser gas introduction passage through the laser gas flow passages back into the laser chamber. As a result, dust particles are prevented from flowing into the magnetic bearings and the motor.
According to an invention described in claim 7, the discharge-pumped excimer laser device according to claim 1, 2, 3, 4, or 5, wherein a differential pressure generating mechanism is disposed in the laser gas flow passages.
With the above invention, since the differential pressure generating mechanism is disposed in the laser gas flow passages, the laser gas is caused to flow reliably from the laser gas introduction passage through the laser gas flow passages back into the laser chamber, and dust particles are prevented from flowing into the housings joined to the opposite sides of the laser chamber. As a result, dust particles are prevented from flowing into the magnetic bearings and the motor.